This invention relates generally to cardiac rhythm management systems and particularly, but not by way of limitation, to a cardiac rhythm management system with atrial shock timing optimization.
When functioning properly, the human heart maintains its own intrinsic rhythm, and is capable of pumping adequate blood throughout the body""s circulatory system. However, some people have irregular cardiac rhythms, referred to as cardiac arrhythmias. Such arrhythmias result in diminished blood circulation. One mode of treating cardiac arrhythmias uses drug therapy. Drugs are often effective at restoring normal heart rhythms. However, drug therapy is not always effective for treating arrhythmias of certain patients. For such patients, an alternative mode of treatment is needed. One such alternative mode of treatment includes the use of a cardiac rhythm management system. Such systems are often implanted in the patient and deliver therapy to the heart.
Cardiac rhythm management systems include, among other things, pacemakers, also referred to as pacers. Pacers deliver timed sequences of low energy electrical stimuli, called pace pulses, to the heart, such as via a transvenous leadwire or catheter (referred to as a xe2x80x9cleadxe2x80x9d) having one or more electrodes disposed in or about the heart. Heart contractions are initiated in response to such pace pulses (this is referred to as xe2x80x9ccapturingxe2x80x9d the heart). By properly timing the delivery of pace pulses, the heart can be induced to contract in proper rhythm, greatly improving its efficiency as a pump. Pacers are often used to treat patients with bradyarrhythmias, that is, hearts that beat too slowly, or irregularly.
Cardiac rhythm management systems also include cardioverters or defibrillators that are capable of delivering higher energy electrical stimuli to the heart. Defibrillators are often used to treat patients with tachyarrhythmias, that is, hearts that beat too quickly. Such too-fast heart rhythms also cause diminished blood circulation because the heart isn""t allowed sufficient time to fill with blood before contracting to expel the blood. Such pumping by the heart is inefficient. A defibrillator is capable of delivering an high energy electrical stimulus that is sometimes referred to as a defibrillation countershock. The countershock interrupts the tachyarrhythmia, allowing the heart to reestablish a normal rhythm for the efficient pumping of blood. In addition to pacers, cardiac rhythm management systems also include, among other things, pacer/defibrillators that combine the functions of pacers and defibrillators, drug delivery devices, and any other systems or devices for diagnosing or treating cardiac arrhythmias.
One problem faced by cardiac rhythm management systems is the proper treatment of atrial tachyarrhythmias, such as atrial fibrillation. Atrial fibrillation is a common cardiac arrhythmia which reduces the pumping efficiency of the heart, though not to as great a degree as in ventricular fibrillation. However, this reduced pumping efficiency requires the ventricle to work harder, which is particularly undesirable in sick patients that cannot tolerate additional stresses. As a result of atrial fibrillation, patients must typically limit their activity and exercise.
Although atrial fibrillation, by itself, is usually not life-threatening, prolonged atrial fibrillation may be associated with strokes, which are thought to be caused by blood clots forming in areas of stagnant blood flow. Treating such blood clots requires the use of anticoagulants. Atrial fibrillation may also cause pain, dizziness, and other irritation to the patient.
An even more serious problem, however, is the risk that atrial fibrillation may induce irregular ventricular heart rhythms by processes that are yet to be fully understood. Moreover, treatment of atrial fibrillation may also induce irregular ventricular heart rhythms. Such induced ventricular arrhythmias compromise pumping efficiency even more drastically than atrial arrhythmias and, in some instances, may be life-threatening. For these and other reasons, there is a need for safe and more effective treatment of atrial fibrillation that avoids inducing ventricular arrhythmias.
The present cardiac rhythm management system provides, among other things, atrial shock timing optimization. The system detects an atrial tachyarrhythmia, such as atrial fibrillation. Such atrial tachyarrhythmias typically cause significant variability in the ventricular heart rate. The present system avoids delivering atrial cardioversion/defibrillation therapy during such irregular ventricular heart activity, because such conditions may be potentially proarrhythmic, such that delivering atrial cardioversion/defibrillation therapy could result in dangerous ventricular arrhythmias. Using Ventricular Rate Regularization (xe2x80x9cVRRxe2x80x9d) techniques described below, the system stabilizes the ventricular heart rate to obtain less potentially proarrhythmic conditions for delivering the atrial tachyarrhythmia therapy. The system withholds delivery of atrial cardioversion/defibrillation therapy until the intervals between ventricular beats (xe2x80x9cV-V intervalsxe2x80x9d) meet certain criteria that decrease the chance that the atrial cardioversion/defibrillation therapy will induce a ventricular arrhythmia.
In one embodiment, the system includes a first method. The first method includes: (a) detecting an atrial tachyarrhythmia, (b) stabilizing a ventricular heart rate at a variable indicated rate based on an underlying intrinsic ventricular heart rate, (c) determining if potentially proarrhythmic conditions exist based on V-V intervals between ventricular events, and (d) delivering cardioversion/defibrillation therapy to the atrium if step (c) indicates no potentially proarrhythmic conditions exist, otherwise withholding the delivery of cardioversion/defibrillation therapy to the atrium until conditions become less potentially proarrhythmic.
In another embodiment, the system includes a second method. The second method includes; (a) obtaining V-V intervals between ventricular beats, (b) computing a first indicated pacing interval based on at least a most recent V-V interval duration and a previous value of the first indicated pacing interval, (c) providing pacing therapy, based on the first indicated pacing interval, (d) detecting a tachyarrhythmia in an atrium, and (e) delivering cardioversion/defibrillation therapy to the atrium.
In another embodiment, the system includes a cardiac rhythm management device. The device includes an atrial heart sensing circuit, a ventricular heart sensing circuit, a ventricular pacing therapy circuit, an atrial cardioversion/defibrillation therapy circuit, and a controller. The controller includes a ventricular rate stabilization module that stabilizes a ventricular heart rate at a variable indicated rate based on an underlying intrinsic ventricular heart rate. The controller also includes an atrial cardioversion/defibrillation control module that (a) determines if potentially proarrhythmic conditions exist based on V-V intervals between ventricular events, and (b) delivers cardioversion/defibrillation therapy to the atrium if conditions become less potentially proarrhythmic, and otherwise withholds the delivery of cardioversion/defibrillation therapy to the atrium. Other aspects of the invention will be apparent on reading the following detailed description of the invention and viewing the drawings that form a part thereof.